Olefins, particularly C2 and C3 olefins, are desirable as a feed source for making derivative products such as oligomers, e.g., higher olefins, and polymers such as polyethylene and polypropylene. Olefin feed sources have traditionally been produced by cracking petroleum feedstocks.
U.S. Pat. No. 5,090,977 discloses a method of making olefins by steam cracking. The method includes separating the olefin product into methane, hydrogen, ethane, ethylene, propylene and C5+ streams. The disclosed separation preferentially produces propylene, and no propane, butane, butene, or butadiene streams are produced.
Oxygenate feed stocks, however, are becoming an alternative to petroleum feed stocks for making olefins, particularly large quantities of ethylene and propylene for the production of higher olefins and plastic materials. In general, the olefins are formed by contacting the oxygenate components with a molecular sieve catalyst to catalytically convert the oxygenates to olefins.
For example, U.S. Pat. No. 4,499,327, discloses a process for making olefins from methanol using any of a variety of silicoaluminophosphate (SAPO) molecular sieve catalysts. The process is carried out at a temperature between 300° C. and 500° C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hr−1. The process is highly selective for making ethylene and propylene.
U.S. Pat. No. 6,121,504 also discloses a method of making olefin product from oxygenate feed using molecular sieve catalysts. Water and other unwanted by-products are removed from the olefin product by contacting with a quench medium. After contacting with the quench medium, a light product fraction is obtained which comprises the desired olefins, but also includes dimethyl ether, methane, CO, CO2, ethane, propane, and other minor components such as water and unreacted oxygenate feedstock.
In order to further process olefins, it is often necessary to reduce or remove undesirable by-products that are present in the olefin composition. For example, U.S. Pat. No. 4,474,647 discloses that dimethyl ether can adversely impact the oligomerization of certain olefins. The patent describes a process for removing dimethyl ether from a C4 and/or C5 olefin stream using distillation. The stream is distilled and separated into an overhead and a bottoms stream. The overhead stream contains dimethyl ether, water, and various hydrocarbons, and the bottoms stream contains purified olefins.
U.S. Pat. No. 5,914,433 discloses a method of making an olefin composition, and a system for removing non-olefin by-products such as CO2. A dewatered olefin composition is washed with caustic to remove CO2, and the washed olefin composition is dried to reduce water added as a result of the caustic wash.
U.S. Pat. No. 5,720,929 discloses a process which includes making isobutylene from isobutane. The isobutylene is cooled and water is stripped from the product. Additional water is removed by washing the product with methanol.
Eng et al., “Integration of the UOP/HYDRO MTO Process into Ethylene Plants,” 10th Ethylene Producers' Conference, 1998, disclose a flow scheme for making an olefin composition from methanol. The flow scheme shows a deethanizer-first flow process.
EP-B1-0 060 103 discloses a process for extracting dimethyl ether from a vapor stream containing ethylene and propylene using a methanol wash system. The methanol wash removes a substantial amount of the dimethyl ether, but also removes a significant amount of the ethylene and propylene.
Additional methods of removing undesirable components from olefin streams are sought. In particular, methods for removing oxygenated hydrocarbons, particularly acetaldehyde, as well as CO2 and water down to the ppm level in olefin product streams, and without removing significant amounts of olefin, are sought.